Known slide fasteners comprise coupling elements formed from a pair of plastic filaments in a helix or meander attached to a pair of confronting support tapes, usually by chain stitches or warp threads. A slider interlocks the coupling elements on the confronting edges. Since the tape is made independently of the coupling elements, allowance must be left for sewing on of the filaments. At present, either the tape is made of continuous pattern of weft and warp threads, or gaps are left in the pattern to be filled in when the element-attaching threads are added.
The resulting fastener has coupling elements with spaced-apart shank portions, leaving room for the attaching threads or the weft. This arrangement lacks stability, since the properties of the fabric and threads affect the alignment of the coupling elements. Stretching or shrinking can occur due to moisture absorption, applied stress, washing or dry cleaning. The bights which connect the coupling elements cannot stabilize these forces, especially since spiral elements have large portions which are free of attaching threads. All these problems are most significant in the very thin plastic filaments commonly employed in the dress industry. Usually the coupling elements are of uniform or equal length.
Present slide-fastener manufacturing processes and apparatus can apply relatively few coupling elements to a given number of warp threads. Automatic warp needles avoid this limitation, but are able to produce slide fastener halves, necessitating a multi-stage process.
More especially, the common helical-coil slide fastener comprises a helix of thermoplastic synthetic-resin monofilament which can form along one side of the helix a multiplicity of coupling elements or heads which are slightly deformed parallel to the axis of the helix so as to interfit or interdigitate with the coupling head of another such coil on the confronting slide fastener half. The coupling head of each turn of the helix is connected by a pair of relatively short shanks to bight portions or bends opposite the coupling head to the shanks of successive turns of the helix. The helix can be somewhat flattened so as to have an elliptical profile as seen along the axis of the helix and the space between each bight and its coupling head is the minimum required to effect coiling of the monofilament.
When such a helix is applied to a woven textile tape, it can receive a filler cord and chain stitching can pass over the shanks and between successive shanks which are spaced apart in accordance with the pitch of the helix to secure the helix to the support tape.
As noted previously it is also possible to "weave" the helix into the support tape directly in which case a loop of at least one and possibly more weft threads passes between each turn of the helix which lies in the manner of a warp within the tape, the coupling heads projecting along an edge of the latter.
There is, therefore, a minimal spacing between each coupling head and the respective bight and a transverse spacing between the successive shanks, even of a single coupling head, which is equal substantially to the pitch of the helix and hence the center-to-center spacing of the coupling heads. Of course, the pitch at any given time is dependent upon the physical parameters of the threads which pass between the shanks, whether these threads are the chain-stitching rhreads or the weft threads which hold the helix in place. The pitch is not, for the most part, completely stable since the spacing between the coupling elements is determined by the textile material interposed between them as noted immediately above. With shrinkage e.g. resulting from the action of moisture, or stretching (e.g. resulting from the application of stress), by the effect of heat and like environmental phenomena, the textile material between the coupling elements varies in dimension and the interlement spacing can vary along the coil or can vary between the two coils. This can interfere with opening and closing of the slide fastener and furthermore limits the closeness with which the coupling elements can be spaced because the minimum spacing is determined by the textile material interposed between these elements.